How do we know Christ's atonement covers all believers?

In John 17:9, Jesus expresses His intention to intercede specifically for those whom the Father has given to Him. This indicates that His death was not just a general provision for all humanity but a specific atonement for His elect. Moreover, 2 Corinthians 5:15 states that Christ died for all so that those who live should no longer live for themselves but for Him. This twofold aspect – Christ's perfect sacrifice and the divine election of the saved – confirms that His atonement effectively secures salvation for every believer, demonstrating the personal nature of God's grace.